Once in a game open up the spawnmenu

then navigate to location of the model either in the default spawn lists or to location of the addon in which the model comes from.

Once you have found the model youd like to make an icon from right click and then click "edit icon" from the drop down menu.

Using the left and right mouse buttons you can drag and rotate the model/camera view to get the perfect shot of the model. If you are having trouble find the model click settings and check the two boxes there.

Once you have found the perfect shot click "Render" and continue to step 4.

If you'd like to size up the icon from the base image size of 64x64 you will need to first, before editing the icon to position it, click resize and select the image size you would like.

Navigate the folder shown:
and locate the image saved here. The image is saved within the spawnicons folder the second you hit "Render"